Process and hardware for deposition of complex thin-film alloys over large areas

ABSTRACT

Systems and methods for depositing complex thin-film alloys on substrates are provided. In particular, systems and methods for the deposition of thin-film Cd 1-x M x Te ternary alloys on substrates using a stacked-source sublimation system are provided, where M is a metal such as Mg, Zn, Mn, and Cu.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a divisional application claiming the benefit ofU.S. patent application Ser. No. 13/733,716 filed Jan. 3, 2013 andentitled “Process and Hardware for Deposition of Complex Thin-FilmAlloys Over Large Areas,” which claims priority to U.S. ProvisionalPatent Application No. 61/582,900 filed Jan. 4, 2012 and entitled“Process and Hardware for Deposition of Complex Thin-Film Alloys OverLarge Areas,” both of which are hereby incorporated herein by referencein their entirety.

GOVERNMENTAL RIGHTS IN THE INVENTION

This invention was made with government support under Grant Nos.IIP0968987 and IIP1127362 awarded by the National Science Foundation.The government has certain rights in the invention.

FIELD OF THE INVENTION

Aspects of the present disclosure relate to systems and methods for thedeposition of complex thin-film alloys on substrates. In particular,aspects of the present disclosure relate to systems and methods for thedeposition of thin-film Cd_(1-x)M_(x)Te ternary alloys on substrates,where M is a metal such as Mg, Zn, Mn, or Cu.

BACKGROUND OF THE INVENTION

Despite recent improvement in efficiency, CdS/CdTe heterojunction solarcells perform significantly below the theoretical limit based on theband gap of CdTe. Several design enhancements to improve the efficiencyof these solar cells have been proposed, including alternative solarcell designs and the incorporation of additional elements into theexisting design of CdS/CdTe solar cells. For example, a tandem junctioncell design which incorporates a CdTe-based ternary alloy with a higherband gap as a p-type absorber in the top cell may exhibit higherefficiency. As another example, the efficiency of the existing CdS/CdTeheterojunction solar cell may be enhanced by the inclusion of anelectron reflector (ER) structure in the form of a CdTe-based ternaryalloy layer. In both cases, the CdTe-based ternary alloy may be a highband gap alloy, such as Cd_(1-x)Mg_(x)Te.

The widespread adoption of these design enhancements of CdS/CdTeheterojunction solar cells may be hampered in part by the lack ofmethods to produce Cd_(1-x)Mg_(x)Te layers of sufficient quality on alarge scale. Existing methods of Cd_(1-x)Mg_(x)Te deposition, includingRF sputtering and co-evaporation methods such as side-by-sideclose-source sublimation (CSS) methods, are known to produce goodquality Cd_(1-x)Mg_(x)Te films, but these existing methods are typicallyslow and demonstrate poor spatial uniformity, rendering these methodsunsuitable for use in large scale manufacturing.

A need exists for improved systems and methods for the deposition ofhigh quality Cd_(1-x)Mg_(x)Te films rapidly, over large areas, and withhigh spatial uniformity. These improved systems and methods wouldeliminate a significant barrier to realizing and commercializing thepotential efficiency improvements to solar cells described above.

BRIEF SUMMARY OF THE INVENTION

This disclosure presents a novel stacked-source sublimation system fordepositing a complex thin-film alloy, including but not limited to aternary Cd_(1-x)M_(x)Te thin film on a substrate.

In a first aspect, a stacked-source sublimation system for thedeposition of a complex thin-film alloy on a substrate is provided. Thesystem includes a first crucible operatively connected to a firstheating element and defining a first heated pocket opening upward at apocket opening. The system also includes a second crucible operativelyconnected to a second heating element and defining a second heatedpocket opening upward into a manifold. The manifold includes a pluralityof conduits that connect the second heated pocket to the first heatedpocket. The system further includes the substrate operatively connectedto a third heating element situated vertically above the pocket opening.

In a second aspect, a stacked-source sublimation system for thedeposition of Cd1-xMgxTe thin-film alloy on a substrate is provided. Thesystem includes a first crucible operatively connected to a firstheating element and defining a first heated pocket opening upward at apocket opening. The first heated pocket contains a first source materialconsisting of CdTe, and the first heating element maintains the firstheated pocket at a first temperature ranging from about 540° C. to about620° C.

In this second aspect, the system also includes a second crucibleconnected to a second heating element. The second crucible is situatedvertically below the first crucible and defines a second heated pocketopening upward into a manifold that includes a plurality of conduits.The second heated pocket contains a second source material consisting ofMg. The second heating element maintains the second heated pocket at asecond temperature ranging from about 350° C. to about 520° C. Theplurality of conduits is distributed evenly over a bottom wall of thefirst heated pocket, and each of the plurality of conduits extendsvertically upward to connect the second heated pocket to the firstheated pocket through the bottom wall.

Also in this second aspect, the system includes the substrateoperatively connected to a third heating element situated verticallyabove the pocket opening at a vertical separation distance of at leastabout 1 μm. The third heating element maintains the substrate at a thirdtemperature ranging from about 300° C. to about 550° C.

In a third aspect, a method for the deposition of a complex thin-filmalloy on a substrate is provided. The method includes providing astacked-source sublimation system that includes a first crucibledefining a first heated pocket opening upward at a pocket opening and asecond crucible operatively connected to a second heating element anddefining a second heated pocket opening upward into a manifoldcomprising a plurality of conduits. The plurality of conduits connectsthe second heated pocket to the first heated pocket.

In this third aspect, the method further includes situating thesubstrate vertically above the pocket opening at a vertical separationdistance of at least about 1 μm, sublimating a first source material ata first temperature within the first heated pocket to form a first flux,and sublimating a second source material at a second temperature withinthe second heated pocket to form a second flux. The method also includestransferring the second flux from the second heated pocket to the firstheated pocket via the manifold to form a deposition mixture, andcontacting the deposition mixture with the substrate to deposit thecomplex thin-film alloy on the substrate. The substrate is maintained ata third temperature in this additional aspect.

In a fourth aspect, a method for the deposition of a Cd1-xMgxTethin-film alloy on a substrate is provided. The method includesproviding a stacked-source sublimation system that includes a firstcrucible defining a first heated pocket opening upward at a pocketopening and a second crucible operatively connected to a second heatingelement and defining a second heated pocket opening upward into amanifold that includes a plurality of conduits. The plurality ofconduits connects the second heated pocket to the first heated pocket.

In this fourth aspect, the method also includes situating a substratevertically above the pocket opening at a vertical separation distance ofat least about 1 μm and sublimating a first source material consistingof CdTe within the first heated pocket at a first temperature rangingfrom about 540° C. to about 620° C. to form a first flux. In addition,the method includes sublimating a second source material consisting ofMg within the second heated pocket at a second temperature ranging fromabout 350° C. to about 520° C. to form a second flux. This methodfurther includes transferring the second flux from the second heatedpocket to the first heated pocket via the manifold to form a depositionmixture and contacting the deposition mixture with the substrate todeposit the complex thin-film alloy on the substrate. The substrate ismaintained at a third temperature ranging from about 400° C. to about550° C.

While multiple embodiments are disclosed, still other embodiments of thepresent disclosure will become apparent to those skilled in the art fromthe following detailed description, which shows and describesillustrative embodiments of the disclosure. As will be realized, thedevices and methods disclosed herein are capable of modifications invarious aspects, all without departing from the spirit and scope of thepresent disclosure. Accordingly, the drawings and detailed descriptionare to be regarded as illustrative in nature and not restrictive.

BRIEF DESCRIPTION OF THE DRAWINGS

The following figures illustrate various aspects of the technologydisclosed herein.

FIG. 1 is a schematic diagram of a stacked-source sublimation system.

FIG. 2 is a graph of the transmission spectra obtained from CdTe andCdMgTe films produced using a prototype stacked-source sublimationsystem.

FIG. 3 is a schematic illustration of a manufacturing system thatincludes a stacked-source sublimation system in an aspect.

FIG. 4 is a graph of the transmission spectra obtained from CdMgTe filmswith varying Mg content produced using a prototype stacked-sourcesublimation system.

FIG. 5 is a graph summarizing the band gaps of Cd_(1-x)Mg_(x)Te filmswith varying Mg content produced using a prototype stacked-sourcesublimation system.

FIGS. 6A-C are scanning electron microscope (SEM) images of threeCd_(1-x)Mg_(x)Te films produced using a prototype stacked-sourcesublimation system with varying Mg content, expressed as x=Mg/(Cd−FMg).FIG. 6A is a SEM image of a CdTe film (x=0) with a band gap of 1.494.FIG. 6B is a SEM image of a film with x=0.156 and a band gap of 1.789.FIG. 6C is a SEM image of a film with x=0.346 and a band gap of 2.125.

FIG. 7 is a graph summarizing the X-ray diffraction (XRD) spectra of thethree Cd_(1-x)Mg_(x)Te films shown in FIGS. 6A-C.

FIG. 8 is a schematic diagram of a stacked-source sublimation system.

FIG. 9 is a flow chart illustrating the steps of a method for depositinga thin-film alloy layer on a substrate using a stacked-sourcesublimation system.

Corresponding reference characters and labels indicate correspondingelements among the views of the drawings. The headings used in thefigures should not be interpreted to limit the scope of the claims.

DETAILED DESCRIPTION

Provided herein are systems and methods for the deposition of complexthin-film alloys using a novel stacked-source sublimation system. Thedesign of the stacked-source sublimation system includes at least twothermally independent sublimation sources that are operatively connectedusing a manifold. The combined fluxes produced by the sublimationsources are mixed and contacted with the substrate, which is alsothermally independent.

The sublimation sources of the stacked-source sublimation system may bearranged in any spatial arrangement without limitation including, butnot limited to, a side-by-side horizontal arrangement and a verticallyaligned arrangement. In one aspect, the stacked-source sublimationsystem may include a top sublimation source and a bottom sublimationsource stacked in a vertically aligned arrangement. In this aspect, thetop and bottom sublimation sources may be connected by a manifold thatincludes one or more conduits through which the bottom flux produced bythe bottom sublimation source may be transported to the substrate alongwith the top flux produced by the top sublimation source. Vapor feedvalves or shutters in the one or more conduits may be included to adjustand/or cut off the feed of the bottom flux as desired to implement avariety of complex thin-film alloy layer structures. In addition, thesublimation source temperatures may be manipulated independently tocontrol individual vapor fluxes as well as to control the overall growthrate of the deposited complex thin-film alloy.

The stacked-source sublimation system overcomes many limitations ofprevious systems for depositing complex thin-film alloys. In particular,this system may be scaled up without compromising layer spatialuniformity to implement the mass production of advanced solar celldesigns and/or any other devices making use of a complex thin-film alloydeposited on a substrate. The manifold of the system may include aplurality of individual conduits arranged in any desired spatialarrangement to enhance the spatial uniformity of the deposited complexthin-film alloy layer. This ability to pattern the conduits of themanifold in any direction to accommodate increases in size of thestacked sublimation sources in a coordinated fashion allows for theimplementation of large area deposition sources and associatedcommercial-scale deposition systems.

In addition, the design of the stacked-source sublimation system maypossess at least several other useful capabilities. The composition offilms deposited using the system may be graded gradually from a firstcomposition to a second composition across the depth of the layer byslowly opening or closing the vapor feed valves supplying fluxesproduced by one or more sublimation sources during deposition. Forexample, gradual grading from CdTe to Cd_(1-x)M_(x)Te may be achieved bygradually opening the vapor feed valves controlling the flux from thebottom (M) sublimation source, where M may be a metal including, but notlimited to Mg. The composition of films deposited using the system mayalso be graded spatially across the exposed area of the deposited layerby opening or closing a portion of the vapor feed valves within one ormore selected regions supplying fluxes produced by the one or moresublimation sources during deposition. For example, a film with acomposition that gradually grades from a first composition at one end ofthe substrate face to a second composition at an opposite end of thesubstrate face may be produced; such a layer may be used to assess theimpact of the layer composition on layer properties such as band width.

The stacked-source sublimation system's design may be compatible with avariety of deposition source materials. Three or more sublimationsources containing different source materials may be used to formthin-film alloys of higher complexity including, but not limited to,ternary alloys and quaternary alloys. For example, an additional dopantsublimation source may be used for slowly adding dopants to thin-filmternary alloys.

Various aspects of the elements of the stacked-source sublimation systemand methods of using the stacked-source sublimation system are describedin further detail herein below.

I. Stacked-Source Sublimation System

In one aspect, illustrated schematically in FIG. 1, the stacked-sourcesublimation system 100 includes a first crucible 102 operativelyconnected to a first heating element 103. The first crucible 102 forms afirst heated pocket 106 containing a first source material 104. Thefirst heated pocket 106 opens upward at a pocket opening 107. The firstheating element 103 maintains the first heated pocket 106 at a firsttemperature independently of the temperatures of other systemcomponents. In an aspect, the first temperature may be sufficiently highto form a first flux by sublimating the first source material 104 withinthe first heated pocket 106.

The stacked-source sublimation system 100 also includes a secondcrucible 114 operatively connected to a second heating element 116. Thesecond crucible 114 forms a second heated pocket 120 containing a secondsource material 118. The second heated pocket 120 opens upward into amanifold 124 connecting the second heated pocket 120 to the first heatedpocket 106. The second heating element 116 maintains the second heatedpocket 120 at a second temperature independently of the temperatures ofother system components.

In an aspect, the second temperature may be sufficiently high for thesublimation of the second source material 118 within the second heatedpocket 120. The second flux resulting from the sublimation of the secondsource material 118 may be carried upward into the first heated pocket106 via the manifold 124 and combined with the first flux to form adeposition mixture. The manifold 124 may include a plurality of verticalconduits 122. These individual conduits 122 may be arranged in anydesired spatial pattern, including, but not limited to a uniformdistribution over the bottom wall 126 of the first crucible 102, asillustrated in FIG. 1; this uniform distribution of the conduits 122 mayenhance the spatial uniformity of the deposition mixture formed by thefirst flux and the second flux within the first heated pocket 106.

In this aspect, a substrate 108 is situated vertically over the pocketopening 107 at a vertical separation distance of at least about 1 μm. Inother aspects, the vertical separation distance may range from about 1μm to about 30 cm. In this position, the substrate 108 contacts thedeposition mixture within the first heated pocket 106. The substrate 108is operatively connected to a third heating element 112 that maintainsthe substrate 108 at a third temperature independently of thetemperatures of other system components.

In one aspect, the third temperature of the substrate 108 may beslightly cooler than the first temperature within the adjacent firstheated pocket 106. This cooler third temperature of the substrate 108may facilitate the formation of the thin-film alloy layer onto thesubstrate 108 while limiting the exposure of the growing layer tothermal stresses that may impact the structural and/or electricalproperties of the layer.

In one other aspect, the stacked-source sublimation system 100 may bysituated within a vacuum device to maintain the system 100 within anoperational atmosphere at a pressure ranging from about 0.1 mTorr toabout 100 mTorr. Any known vacuum device may be used to produce theoperational atmosphere within which the system 100 is situated.Non-limiting examples of suitable vacuum devices include rotary vanepumps, reciprocating piston pumps, rotary piston pumps, scroll pumps,screw pumps, rotary lobe pumps, molecular drag pumps, and anycombination thereof. The vacuum device may further include one or morefiltration devices operationally connected to the exhaust of the vacuumdevice to capture vapor byproducts of the deposition process prior todischarging the exhaust into the external atmosphere. Non-limitingexamples of suitable filtration devices include cold traps, filters,scrubbers, and any combination thereof.

In this other aspect, the operational atmosphere may include one or moregases of any composition without limitation. Non-limiting examples ofcompositions of gases suitable for inclusion within the operationalatmosphere include inert gases such as Ar, N₂, and He, reactive gasessuch as O₂ and H₂, and any combination thereof. In another additionalaspect, the operational atmosphere is an inert gas atmosphere comprisingone or more inert gases including, but not limited to: Ar, N₂, He, andany combination thereof.

a. First Crucible and First Heating Element

Referring again to FIG. 1, the stacked-source sublimation system 100includes the first crucible 102 and associated first heating element103. The first crucible 102 defines the first heated pocket 106containing the first source material 104 which is sublimated within thefirst heated pocket 106 to produce the first flux. The first fluxcombines with the second flux generated within the second crucible 114to produce the deposition mixture. The deposition mixture rises andexits the first heated pocket 106 via the pocket exit 107.

The first crucible 102 is designed to provide an evenly distributeddeposition mixture to the substrate surface in order to deposit thethin-layer alloy in a spatially uniform manner. In addition, the heatedpocket design of the first crucible 102 facilitates the rapid depositionof alloy layers at growth rates as high as 1.0 μm/min or more. Thisheated pocket design may be easily scaled up or down as needed, makingpossible the deposition of highly uniform thin-layer alloys at acommercial manufacturing scale for the production of devices including,but not limited to, solar cells.

i. First Crucible

In an aspect, the first crucible 102 includes a bottom wall 126 and atleast one side wall 128. The bottom wall 126 and at least one side wall128 together form a continuous surface defining the first heated pocket106. The bottom wall 126 and at least one side wall 128 may beconstructed using any suitable material which has an acceptable level ofthermal conductivity. These suitable materials may further have a lowlevel of porosity to prevent the adsorption of air and water vapor andmay contain suitable low levels of impurities. Non-limiting examples ofsuitable materials for the construction of the first crucible 102include graphite materials such as purified pyrolytic grade graphite,metals, metals coated with ceramics, and any other known crucibleconstruction material.

In one aspect, the first crucible 102 is constructed from a graphitematerial. Graphite has high thermal emissivity characteristics, enablingthe rapid heating of the first source material 104 within the firstheated pocket 106. In addition, graphite has relatively high thermalconductivity characteristics, resulting in a uniform temperaturedistribution throughout the first heated pocket 106 within the firstcrucible 102.

The overall dimensions of the bottom wall 126 may be any size or shapewithout limitation. In one aspect, the bottom wall 126 may beessentially matched in size and shape to the surface of the substrate108 and is typically a flat horizontal structure. The first heatingelement 103 may be situated in close proximity and/or attached to thebottom wall 126 as described herein below.

The bottom wall 126 further defines a plurality of openings 130 throughwhich the second flux produced by the second crucible 114 may pass fromthe second heated pocket 120 to the first heated pocket 106. In oneaspect, the plurality of openings 130 may connect to the plurality ofconduits 122 of the manifold 124 as illustrated in FIG. 1. In anotheraspect, the plurality of openings 130 may not be directly connected to aconduit or manifold 124; instead the second flux produced within thesecond heated pocket 120 may passively rise vertically through theplurality of openings 130.

The plurality of openings 130 may be of any cross-sectional size andshape and may be of any number and distribution without limitation. Inan aspect, the number and distribution of openings 130 may be designedto provide a spatially uniform distribution of the second fluxtransferred from the second heated pocket 120 into the first heatedpocket 106. In one aspect, the openings 130 may be uniformly distributedthroughout the bottom wall 126. In another aspect, the openings 130 maybe asymmetrically distributed to compensate for non-ideal effects suchas the effects of the side walls 128 and corners on the flow of thedeposition mixture within the first heated pocket 106.

In an aspect, the bottom wall 126 may include textures, indentations, orother features to facilitate the desired distribution of the firstsource material 104 within the first heated pocket 104. In one aspect,the bottom wall 126 may include raised ridges to facilitate thesituating of the first source material 104 at selected regions of thebottom wall 126. In this aspect, the raised ridges may form closedshapes including, but not limited to circles, polygons, and any otherclosed shape to contain a small amount of the first source material 104within the region of the closed shape. In another aspect, the bottomwall 126 may contain one or more indentations such as wells formed intothe surface of the bottom wall 126. In these various aspects, thetextures, indentations, or other features may have any distribution overthe area of the bottom wall 126 without limitation.

The one or more side walls 128 are typically oriented vertically andperpendicular to the bottom wall 126, as illustrated in FIG. 1. Thisvertical orientation facilitates the collimation of the upward flow ofdeposition mixture, as described herein below. As a result of thisvertical wall orientation, the pocket opening 107 formed by the upperedges of the one or more side walls 128 is essentially the same size andshape as the bottom wall 126.

The dimensions of the first crucible 102 may be influenced by any one ormore of at least several factors. The pocket opening 107 of the firstcrucible 102 may be essentially matched in size and shape to the exposedsurface of the substrate 108. In addition, the height of the one or moreside walls 128 of the first crucible 102 may be sized in order toenhance the function of the first crucible 102 as a heated pocketdeposition device, as described herein below.

ii. First Heated Pocket

The first heated pocket 106 formed within the bottom wall 126 and one ormore side walls 128 of the first crucible 102: i) receives the first andsecond fluxes formed by the sublimation of the first source material 104and second source material 118, respectively, ii) mixes the two fluxesto form a deposition mixture, and iii) delivers the deposition mixturein a spatially uniform distribution to the substrate 108 for deposition.The overall design of the first heated pocket 106 may be based in parton the heated pocket deposition (HPD) devices as described in U.S. Pat.No. 6,423,565, which is hereby incorporated by reference in itsentirety. In brief, the heated pocket design makes possible the uniformdeposition of a sublimated source material over a substrate surface.

Referring back to FIG. 1, the substrate 108 is situated vertically abovethe pocket opening 107 and is separated from the first crucible 102 by asmall vertical separation distance. This vertical separation may preventthe substrate 108 from touching the first crucible 102 and damaging thedeposited alloy films while maintaining a sufficient concentration ofdeposition mixture within the first heated pocket 106 for deposition atthe desired growth rate and uniformity. In one aspect, the verticalseparation distance may be sufficiently small to provide a closetolerance slip fit seal between the top of the first crucible 102 andthe substrate 108. Any vapor leak through this vertical separation inthis aspect will be in the molecular flow regime, effectively allowingthe substrate 108 to act as a shutter across the first heated pocket106.

This shuttering effect of the substrate 108 may enhance the uniformheating of the deposition mixture within the first heated pocket 106.The uniform and high temperature of the deposition mixture within thefirst heated pocket 106 may inhibit the formation of undesirednanoparticles within the deposition mixture due to gas scatteringcollisions as described herein below.

In another aspect, the first heated pocket 106 may be at least partiallyopen during both film deposition and in-between deposition on successivesubstrates 102. Without being limited to any particular theory, byallowing excess deposition mixture material to leave the first heatedpocket 106, the sublimation rates of the first source material 104 andthe second source material 118 may be maintained, resulting inrepeatable deposition characteristics over multiple substrates 108.

In various aspects, the vertical separation distance between thesubstrate 108 and the pocket opening 107 may be at least about 1 μm. Invarious other aspects, the vertical separation distance may range fromabout 1 μm to about 30 cm. This vertical separation distance may varybased on the overall size of the stacked-source sublimation system 100as well as any one or more of the considerations described hereinpreviously. In one aspect, if the deposition surface of the substrate108 is about 10 cm×10 cm, the vertical separation distance may rangefrom about 2 mm and about 8 mm. In another aspect, if the depositionsurface of the substrate 108 is about 10 cm×10 cm, the verticalseparation distance may be about 4 mm.

In an additional aspect, the at least one side wall 128 of the firstheated pocket 106 may collimate the vapor flux from the subliming firstsource material 104 and second source material 118. Since the verticalseparation between the substrate 108 and the first crucible 102 is atthe top of the first heated pocket 106 and at a right angle to thecollimated vapor flux, nearly all of the vapor flux will pass thisvertical gap without directly entering this gap. Any vapor which doesenter this gap due to gas scattering will be readily deposited on thesurface of the substrate 108.

The distance between the first source material 104 and the substrate108, or alternatively the depth of the first heated pocket 106, isgoverned by the height of the one or more side walls 128 of the firstcrucible 102. In one aspect, the depth of the first heated pocket 106may be sufficient to allow for gas scattering of the deposition mixturewithin a desired range.

Without being limited to any particular theory, gas scattering is theresult of collisions among the molecules of the deposition mixture orbetween molecules of the deposition mixture and the molecules of theambient background gas. These collisions deflect and scatter themolecules of the deposition mixture, thereby causing a deviation from astraight vertical path from the first source material 104 to thesubstrate 108. This scattering of the molecules of the depositionmixture facilitates uniform deposition on the substrate 108. The degreeof gas scattering may be expressed in terms of the Knudsen number,defined as the ratio of the mean free path of the vapor flux in thefirst heated pocket 106 at a given temperature and pressure, divided bythe vertical height of the first heated pocket 106 (i.e., the distancebetween the first source material 104 and the substrate 108). If theKnudsen number is less than 0.01, then the molecules of the depositionmixture within the first heated pocket 106 move in a viscous flow regimeand gas scattering will be a significant factor. In the viscous flowregime, this significant gas scattering may lead to such a loss ofenergy from the molecules of the deposition mixture that they condenseto form undesired nanoparticles. For Knudsen numbers greater than 1, themolecules of the deposition mixture move in a molecular flow regime withvery little gas scattering. In this molecular flow regime, the moleculesof the deposition mixture travel in straight vertical lines to thesubstrate 108, typically resulting in non-uniform film thickness acrossthe substrate 108. For Knudsen numbers between 0.01 and 1, the moleculesof the deposition mixture move in a transition flow regime with some gasscattering. In the transition flow regime, the vapor flux is randomizedby gas scattering, but does not condense into undesired nanoparticlesbefore depositing on the substrate 108.

In one aspect, the depth of the first heated pocket 106 results indeposition at Knudsen numbers in the transition regime from 0.07 to0.44, resulting in the deposit of a uniform film thickness across thesubstrate 108.

iii. First Heating Element

The first heating element 103 associated with the first crucible 102maintains the temperature of the first heated pocket 106 at a firsttemperature independently of other elements of the stacked-sourcesublimation system 100 including, but not limited to the substrate 108and the second crucible 114. The first temperature may be selected basedon any one or more of at least several factors. The first temperaturemay be selected to be sufficiently high for the sublimation of the firstsource material 104 at a desired sublimation rate; this sublimation ratemay be further influenced by the ambient pressure within which thesystem 100 operates. In addition, the first temperature may be selectedto inhibit the formation of undesired nanoparticles as the depositionmixture is formed and transported to the surface of the substrate 108.The first temperature may be selected to be higher than the thirdtemperature at which the substrate 108 is maintained to enhance thedeposition of the thin-layer alloy onto the substrate 108.

The first temperature may be manipulated to control the process ofdepositing a thin-film alloy layer on a substrate 108 using thestacked-source sublimation system 100. In one aspect, a higher firsttemperature may result in a higher growth rate of the thin-film alloylayer on the substrate 108. In another aspect, a higher firsttemperature may result in a thin film alloy layer containing a smallerproportion of material associated with the sublimation of the firstsource material 104. For example, if the first source material 104 isCdTe and the second source material 118 is Mg, a higher firsttemperature may result in a higher proportion of Cd in theCd_(1-x)Mg_(x)Te layer.

In an aspect, the stacked-source sublimation system 100 may additionallyinclude a first temperature sensor to monitor the first temperaturewithin the first heated pocket 106. The output of the first temperaturesensor may be used as feedback to a control system used to control thefirst heating element 103 including, but not limited to: a proportionalwith integral and derivative (PID) controller or any other suitablecontroller. Non-limiting examples of suitable first temperature sensordevices include: a thermocouple and an infrared temperature sensor.

Any known heating device may be used as the first heating element 103without limitation. Non-limiting examples of devices suitable for use asa first heating element 103 include: resistive heating devices such asNiCr coils, inductive heating devices, and radiative heating devicessuch as quartz-halogen lamps. In one aspect, the first heating element103 may be one or more resistive NiCr coils. In this aspect, the NiCrwire may be embedded into the material of the first crucible 102 usingan embedding material including, but not limited to, an alumina-basedceramic material. In another aspect, the first crucible 102 mayincorporate a plasma generation device (not shown) to provide anadditional degree of control over the properties of the thin-film alloysproduced by the system 100.

In an aspect, the first temperature may range from about 100° C. toabout 1000° C. In another aspect, if the first source material 104 isCdTe, the first temperature may range from about 540° C. to about 600°C. In this aspect, the first temperature may be influenced by thepressure at which the system 100 is operated as well as other processparameters as described herein above.

iv. First Source Material

The first source material 104 is heated to the first temperature withinthe first heated pocket 106, resulting in the formation of the firstflux by sublimation. Typically, the first source material 104 isprovided in a particulate or powdered form. In one aspect, the firstsource material 104 is distributed in an evenly spaced pattern acrossthe floor of the first heated pocket 106 formed by the bottom wall 126of the first crucible 102. As described herein previously, the bottomwall 126 may include indentations such as wells and/or raised texturesto facilitate the even distribution of the first source material 104within the first heated pocket 106.

Any known form of the first source material 104 may be used withoutlimitation including, but not limited to: powders, pellets pressed frompowder, and/or random chunks. In one aspect, the first source material104 may be provided in the form of chunks. In another aspect, the chunksmay be standardized in size in order to enhance the uniformity ofsublimation during deposition. In this other aspect, a quantity ofchunks may be processed using known methods including but not limited tosieving to select a sub-quantity of chunks falling within a desired sizerange. In yet another aspect, the first source material 104 may beprovided in the form of chunks with diameters ranging between about0.067 inches (+12 mesh) and about 0.25 inches.

The composition of the first source material 104 may be any knownmaterial capable of being vaporized at a temperature range correspondingto the first temperature as described previously herein withoutlimitation. In various aspects, the composition of the first sourcematerial 104 may be any semiconductor material formed by any combinationof the elements Zn, Cd, Hg, S, Se, or Te, elements from group IIB andgroup VIB of the periodic table. In various other aspects, thecomposition of the first source material 104 may be compound chosen fromtelluride compounds including, but not limited to CdTe; sulfidecompounds including, but not limited to CdS; chloride compoundsincluding, but not limited to CdCl₂, CuCl₂, and MgCl₂, and selenidecompounds. In one aspect, the composition of the first source material104 may be CdS or CdTe. In another aspect, the composition of the firstsource material 104 is CdTe.

The purity of the first source material 104 may be sufficiently high toavoid the incorporation of unwanted elements into the depositedthin-film alloy and to minimize undesired electrical characteristics. Inone aspect, the purity of the first source material 104 is at least99.9%. In other aspects, the purity of the first source material 104 isat least 99.95%, at least 99.99%, at least 99.995%, and at least99.999%.

b. Second Crucible, Second Heating Element, and Manifold

Referring again to FIG. 1, the stacked-source sublimation system 100includes the second crucible 114 and associated second heating element116. The second crucible 114 defines the second heated pocket 120containing the second source material 118 which is sublimated within thesecond heated pocket 120 to produce the second flux. The second fluxproduced by the second crucible 114 is transferred to the first heatedpocket 106 via a manifold 124 connecting the second heated pocket 120 tothe first heated pocket 106. The second flux combines with the firstflux generated within the first crucible 102 to produce the depositionmixture.

The second crucible 114 is designed to provide an evenly distributedtransfer of the second flux to the first heated pocket 106, resulting inthe deposition of the thin-layer alloy in a spatially uniform manneronto the substrate 108. In addition, the second crucible 114 ismaintained at a second temperature independently of other elements ofthe system 100. This ability to control the temperature of the secondheated pocket 120 provides one means of controlling the sublimation rateof the second source material 118 and by extension the rate of transferof the second flux to the first heated pocket 106. The second crucible114 may further include one or more flux control elements to increase ordecrease the rate of transfer of the second flux.

This control of the rate of transfer of the second flux provides theability to control the degree of inclusion of the elements from thesecond source material 118 in the thin-film alloy deposited on thesubstrate 108. This degree of inclusion may be manipulated during thedeposition process to form layers with abrupt changes in compositionand/or gradually graded changes in composition as desired.

The second crucible 114 makes use of a heated pocket design similar tothe design of the first crucible 102. The size, shape and otherdimensions and operational parameters of the second crucible 114 may bevaried independently of the other elements of the system 100 including,but not limited to, the first crucible 102. In one aspect, the secondcrucible 114 may be increased or decreased in size in proportion tochanges in the size of the first crucible 102. In this aspect, thecoordinated changes in size of the first crucible 102 and secondcrucible 114 make possible the scaling of the stacked-source sublimationsystem 100 up or down in size with minimal impact on the uniformity ofthe thin-film alloys produced by the system 100.

In another aspect, stacked-source sublimation system 100 may include twoor more second crucibles 114. In this aspect, the additional secondcrucibles 114 may be added or subtracted in proportion to changes in thesize of the first crucible 102. The inclusion of the two or more secondcrucibles 114 may enhance the degree of control over the uniformity ofthe second flux produced in this aspect.

i. Second Crucible

In various aspects, the second crucible 114 is similar in design to thedesign of the first crucible 102. In an aspect, the second crucible 114includes a bottom wall 132 and at least one side wall 134. The bottomwall 132 and at least one side wall 134 together form a continuoussurface defining the second heated pocket 120. Unlike the first heatedpocket 106 which opens upward at the pocket opening 107, the secondheated pocket 120 is an enclosed volume in various aspects. Thisenclosed volume of the second heated pocket 120 provides an enhanceddegree of control over the rate of production and transfer of the secondflux, thereby enhancing the capability to control the degree ofinclusion of the elements contained in the second flux within thedeposited thin-layer alloy as described previously herein.

In one aspect, the second heated pocket 120 is enclosed by the bottomwall 132, the at least one side wall 134, as well as a manifold 124which forms an upper boundary of the second heated pocket 120 asillustrated in FIG. 1. In another aspect, illustrated in FIG. 8, theupper boundary of the second heated pocket 120 is formed by the bottomwall 126 of the first crucible 102; in this aspect, the side walls 134extend upward and are sealed to the bottom wall 126 of the firstcrucible 102.

In another aspect, a heat shield 138 may be situated between the firstcrucible 102 and the second crucible 114. In this aspect, the heatshield 138 may thermally separate the first crucible 102 and the secondcrucible 114, thereby enhancing the ability to regulate independentlythe first temperature within the first heated pocket 106 and the secondtemperature within the second heated pocket 120. In this aspect, theheat shield 138 may be produced using any one or more known heatshielding materials including, but not limited to metals, ceramics,and/or any other known heat shield material. The heat shield 138 may beprovided in the form of a single material layer, or the heat shield 138may be provided in the form of a composite structure containing two ormore layers of different materials. In an additional aspect, the heatshield 138 may a composite structure containing multiple alternatinglayers of alumina ceramic sheets and stainless steel foil sheets.

The bottom wall 132, and the at least one side wall 134 may beconstructed using any suitable material which has an acceptable level ofthermal conductivity similar to those materials described for theconstruction of the first crucible 102. These suitable materials mayfurther have a low level of porosity to prevent the adsorption of airand water vapor and may contain suitable low levels of impurities.Non-limiting examples of suitable materials include graphite materialssuch as purified pyrolytic grade graphite, metals, metals coated withceramics, and any other known crucible construction material.

In one aspect, the second crucible 114 is constructed from a graphitematerial. Graphite has high thermal emissivity characteristics, enablingthe rapid heating of the second source material 118 within the secondheated pocket 120. In addition, graphite has relatively high thermalconductivity characteristics, resulting in a uniform temperaturedistribution throughout the second heated pocket 120.

The overall dimensions of the bottom wall 132 and the at least one sidewall 134 may be any size or shape without limitation. In one aspect, thebottom wall 132 may be essentially matched in size and shape to the sizeand shape of the bottom wall 136 of the first crucible 102 and istypically a flat horizontal structure. The second heating element 116may be situated in close proximity and/or attached to the bottom wall132 as described herein below.

In an aspect, the bottom wall 132 may include textures, indentations, orother features to facilitate the desired distribution of the secondsource material 118 within the second heated pocket 120. In one aspect,the bottom wall 132 may include raised ridges to facilitate thesituating of the second source material 118 at selected regions of thebottom wall 132. In this aspect, the raised ridges may form closedshapes including, but not limited to circles, polygons, and any otherclosed shape to contain a small amount of the second source material 118within the region of the closed shape. In another aspect, the bottomwall 132 may contain one or more indentations such as wells formed intothe surface of the bottom wall 132. In these various aspects, thetextures, indentations, or other features may have any distribution overthe area of the bottom wall 132 without limitation.

The at least one side wall 134 is typically oriented vertically andperpendicular to the bottom wall 132, as illustrated in FIG. 1. Thisvertical orientation facilitates the collimation of the upward flow ofthe second flux, as described previously herein in connection with thedesign of the first crucible 102.

The dimensions of the second crucible 114 may be influenced by any oneor more of at least several factors. The top of the second crucible 114may be essentially matched in size and shape to the bottom of the firstcrucible 102. In addition, the height of the at least one side wall 134may be sized in order to enhance the function of the second crucible 114as a heated pocket deposition device, as described herein previously.For example, the at least one side wall 134 may be dimensioned such thatthe second crucible functions at a Knudsen number ranging from about0.07 to about 0.44 as described herein previously.

ii. Second Heated Pocket

The second heated pocket 120 formed within the bottom wall 132 and atleast one side wall 134 of the first crucible 114: i) produces thesecond flux formed by the sublimation of the second source material 118and ii) delivers the second flux to the first heated pocket 106 at acontrolled rate. The overall design of the second heated pocket 114 maybe similar to the design of first heated pocket 106 with onedistinction: the second heated pocket 114 is a closed volume.

The closed volume design of the second heated pocket 114 may enhance theuniform heating of the second flux within the second heated pocket 114.The uniform and high temperature of the second flux within the secondheated pocket 114 may inhibit the formation of undesired nanoparticleswithin the second flux due to gas scattering collisions as describedherein previously. In an additional aspect, the at least one side wall134 of the first heated pocket 106 may collimate the second flux fromthe subliming second source material 118.

The distance between the second source material 118 and the bottom ofthe first crucible 102, or alternatively the depth of the second heatedpocket 120, is governed by the height of the at least one side walls 134of the second crucible 114. In one aspect, the depth of the secondheated pocket 120 may be sufficient to allow for gas scattering of thesecond flux within a desired range as described previously herein.

iii. Manifold and Flux Control Elements

In various aspects, the second flux formed by the sublimation of thesecond source material 118 within the second heated pocket 120 may betransferred at a controlled rate to the first heated pocket 106 by avariety of means. In one aspect, the second flux may be transferred byway of a plurality of openings 130 formed through the bottom wall 126 ofthe first crucible 102, as illustrated in FIG. 8. In this aspect, thesecond flux may passively rise and pass from the second heated pocket120 to the first heated pocket 106 by way of the openings 130. Asdescribed herein previously, the openings may have any size, shape,and/or spatial distribution without limitation. In an aspect, the size,shape, and spatial distribution of the openings 130 may be configured toprovide a spatially uniform transfer of the second flux into the firstheated pocket 106.

In another aspect, the second flux may be transferred at a controlledrate to the first heated pocket 106 through a manifold 124 asillustrated in FIG. 1. In this aspect, the manifold 124 may include aplurality of conduits 122 connected to the plurality of openings 130 onthe bottom wall 126 of the first crucible 102. The length,cross-sectional shape and dimensions, number, and spatial distributionof the plurality of openings 130 may vary without limitation. Typically,the particular configuration of the conduits 122 of the manifold 124 aredesigned based on any one or more of at least several considerationsincluding, but not limited to: inhibition of nanoparticle formation;uniform transfer of second flux into first heated pocket 106, andcontrol of rate of transfer of second flux. For example, the combinedcross-sectional area of all conduits 122 of a manifold 124 may influencethe capacity of the manifold 124 to transfer the second flux to thefirst heated pocket 106.

In various aspects, the rate of transfer of the second flux may beregulated by manipulation of the second temperature at which the secondheated pocket 120 is maintained. For example, if the second temperatureis increased, the sublimation rate of the second source material 118 isincreased, with a corresponding increase in the rate of transfer of thesecond flux. In other aspects, additional flux control elements may beincorporated into the second crucible including, but not limited to:adjustable flow valves, adjustable shutter plates, and any othersuitable flow control device. The additional flux control elements maybe used in lieu of, or in addition to, the manipulation of the secondtemperature to control the rate of transfer of the second flux.

In one aspect, the plurality of adjustable valves 136 may be operativelyconnected to the plurality of conduits 122 of the manifold 124 asillustrated in FIG. 1. In this aspect, the plurality of adjustablevalves 136 may occlude the plurality of conduits 122 when the valves 136are adjusted to a closed position. In an open position, the valves 136permit the unimpeded transfer of the second flux through the conduits122.

In another aspect, an adjustable shutter plate 136 may be operativelyconnected to the second crucible 114 to regulate the rate of transfer ofthe second flux as illustrated in FIG. 8. In this aspect, the shutterplate 136 may be a thin plate containing a plurality of holes 138corresponding to the size, shape, and spatial distribution of theplurality of openings in the bottom wall 126 of the first crucible 102.When the shutter plate 136 is adjusted to an open position, asillustrated in FIG. 8, the plurality of holes 138 are aligned with theplurality of openings 130, permitting the unimpeded transfer of thesecond flux into the first heated pocket 106. When the shutter plate 136is adjusted to a closed position by sliding to the left or right alongthe bottom wall 126, the plurality of holes 138 are shifted out ofalignment with the plurality of openings 130, resulting in the occlusionof the openings 130 and blocking the transfer of the second flux intothe first heated pocket 106.

In yet another aspect, the additional flux elements may be operated toeither permit or block the transfer of the second flux in an “on-off”manner. In another additional aspect, the additional flux elements maybe gradually adjusted during deposition to gradually increase ordecrease the rate of transfer of the second flux. In this manner, theadditional flux elements may be used to produce thin-film alloys withvariable compositions including, but not limited to an abrupt change incomposition, a gradual change in composition, or a combination thereof.

iv. Second Heating Element

The second heating element 116 associated with the second crucible 114maintains the temperature of the second heated pocket 120 at a secondtemperature independently of other elements of the stacked-sourcesublimation system 100 including, but not limited to the substrate 108and the first crucible 102. The second temperature may be selected basedon any one or more of at least several factors. The second temperaturemay be selected to be sufficiently high for the sublimation of thesecond source material 118 at a desired sublimation rate; thissublimation rate may be further influenced by the ambient pressurewithin which the system 100 operates. In addition, the secondtemperature may be selected to inhibit the formation of undesirednanoparticles as the second flux is formed and transported to the firstheated pocket 106. The second temperature may be selected to be lowerthan the first temperature at which the first heated pocket 106 ismaintained to inhibit the condensation of the second flux within thefirst heated pocket 106 during the mixing of the first flux and thesecond flux to form the deposition mixture, and may further inhibit theformation of undesired nanoparticles within the first heated pocket 106.

The second temperature may be manipulated to control the process ofdepositing a thin-film alloy layer on a substrate 108 using thestacked-source sublimation system 100. In one aspect, a higher secondtemperature may result in a higher proportion of second flux elements inthe deposition mixture and by extension a higher proportion of thesesecond flux elements in the deposition thin-film alloy layer. Forexample, if the first source material 104 is CdTe and the second sourcematerial 118 is Mg, a higher second temperature may result in a higherproportion of Mg in the Cd_(1-x)Mg_(x)Te layer.

In an aspect, the stacked-source sublimation system 100 may additionallyinclude a second temperature sensor to monitor the second temperaturewithin the second heated pocket 120. The output of the secondtemperature sensor may be used as feedback to a control system used tocontrol the second heating element 116 including, but not limited to: aproportional with integral and derivative (PID) controller or any othersuitable controller. Non-limiting examples of suitable secondtemperature sensor devices include: a thermocouple and an infraredtemperature sensor.

Any known heating device may be used as the second heating element 116without limitation. Non-limiting examples of devices suitable for use asa second heating element 116 include: resistive heating devices such asNiCr coils, inductive heating devices, and radiative heating devicessuch as quartz-halogen lamps. In one aspect, the second heating element116 may be one or more resistive NiCr coils. In this aspect, the NiCrwire may be embedded into the material of the second crucible 114 usingan embedding material including, but not limited to, an alumina-basedceramic material.

In one aspect, the second temperature may range from about 100° C. toabout 1000° C. during deposition. In another aspect, if the secondsource material 118 is Mg, the second temperature may range from about350° C. to about 520° C. during deposition. In this aspect, the secondtemperature may be influenced by the pressure at which the system 100 isoperated as well as other process parameters as described herein above.

In additional aspects, if the second source material 118 is prone to theformation of an oxidation layer, this oxidation layer may be removedprior to the initiation of deposition using any known method including,but not limited to forming a plasma from the second source material 118,sputtering the second source material 118 in situ, and preheating thesecond source material 118 to burn off the residual oxide layer. In oneadditional aspect, the second crucible 114 may be heated to an elevatedtemperature prior to the initiation of deposition in order to burn offany oxidation layer on the second source material 118. For example, ifthe second source material 118 is Mg metal, the second crucible 114 maybe heated to a second temperature of about 520° C. to remove anyresidual MgO that may have formed on the surface of the Mg sourcematerial.

v. Second Source Material

The second source material 118 is heated to the second temperaturewithin the second heated pocket 120, resulting in the formation of thesecond flux by sublimation. Typically, the second source material 118 isprovided in a particulate or powdered form. In one aspect, the secondsource material 118 is distributed in an evenly spaced pattern acrossthe floor of the second heated pocket 120 formed by the bottom wall 132of the second crucible 114. As described herein previously, the bottomwall 132 may include indentations such as wells and/or raised texturesto facilitate the even distribution of the second source material 118within the second heated pocket 120.

Any known form of the second source material 118 may be used withoutlimitation including, but not limited to: powders, pellets, pelletspressed from powder, and/or random chunks. In one aspect, the secondsource material 118 may be provided in the form of pellets. In anotheraspect, the pellets may be standardized in size in order to enhance theuniformity of sublimation during deposition. Sublimation-grade pelletssuitable for use as a second source material 118 are readily availablethrough commercial sources.

The composition of the second source material 118 may be any knownmaterial capable of being vaporized at a temperature range correspondingto the second temperature as described previously herein withoutlimitation. In various other aspects, the composition of the secondsource material 118 may be a metal chosen from Mg, Zn, Mn, Cu, Hg, Bi,Pb, and Cd or a compound chosen from telluride compounds including, butnot limited to CdTe; sulfide compounds including, but not limited toCdS; chloride compounds including, but not limited to CdCl₂, CuCl₂, andMgCl₂; and selenide compounds. The choice of second source material 118may be based on the composition of the first source material 104, thedesired composition of the thin-film alloy, and the desired electricalproperties of the thin-film alloy such as band gap. In one aspect, thecomposition of the second source material 118 is Mg.

In various additional aspects, the second source material 118 may be anysemiconductor material formed by any combination of the elements Zn, Cd,Hg, S, Se, or Te, elements from group IIB and group VIB of the periodictable, or any other suitable material. Non-limiting examples of suitablesemiconductor materials and other suitable materials include: CdS, Cd,Te, CdCl₂, and MgCl₂.

In one additional aspect, the composition of the second source material118 may be CdS. In this one additional aspect, if the composition of thefirst source material 104 is CdTe, the composition of the depositedthin-film alloy layer may be CdS_(x)Te_(1-x).

The purity of the second source material 118 may be sufficiently high toavoid the incorporation of unwanted elements into the depositedthin-film alloy and to minimize undesired electrical characteristics. Inone aspect, the purity of the second source material 118 is at least99.9%. In other aspects, the purity of the second source material 118 isat least 99.95%, at least 99.99%, at least 99.995%, and at least99.999%.

c. Substrate and Third Heating Element

Referring back to FIG. 1, the substrate 108 is situated vertically abovethe pocket opening 107 of the first crucible 102. The deposition mixtureproduced within the first crucible 102 exits the pocket opening 107 andforms the thin-film alloy layer upon contact with the substrate surface.The third heating element 112 maintains the substrate 108 at a thirdtemperature which is typically cooler than the first temperature atwhich the deposition mixture is maintained within the first crucible102. This cooler temperature may enhance the deposition of thedeposition mixture onto the substrate 108.

i. Substrate

In various aspects, the substrate 108 is a thin plate that is situatedhorizontally and in vertical alignment with the pocket opening 107.Although the substrate 108 is typically planar, slightly curvedsubstrate geometries may also be compatible with the system 100. Thesubstrate 108 is situated slightly above the pocket opening 107 at aslight vertical separation distance, as described herein previously, toenhance the uniformity of the thin-film alloy layer deposited on thesubstrate 108.

The substrate may be made of any suitable material capable ofwithstanding the elevated temperatures at which the system 100 operates,which may be as high as 620° C. within the first heated pocket 106. Invarious aspects, the substrate may be any suitable material includingbut not limited to: glass materials such as soda-lime glass;semiconductor materials such as CdTe, CdS, Si, and Ge; metals; and anyother suitable substrate material.

ii. Third Heating Element

In various aspects, the substrate 108 may be maintained at a thirdtemperature using a third heating element 112. The third heating element112 maintains the substrate 108 at this third temperature independentlyof other elements of the stacked-source sublimation system 100including, but not limited to the first crucible 102. The thirdtemperature may be selected based on any one or more of at least severalfactors. The third temperature may be selected to be lower than thefirst temperature at which the deposition mixture is maintained in thefirst crucible 102; the cooler substrate surface may enhance thedeposition of the thin-film alloy onto the substrate 108. The thirdtemperature may be further selected to be sufficiently low so as toinhibit the reaction of the deposited thin-film layer with the materialof the underlying substrate 108. The third temperature may further beselected to be high enough to inhibit the formation of defects duringdeposition due to thermal stress effects. In addition, the thirdtemperature may influence the growth rate of the deposited thin-filmalloy layer, and well as various physical and/or electrical propertiesof the deposited thin-film alloy layer including, but not limited to:grain size, surface roughness, orientation, film doping, and band gap.

In an aspect, a heat transfer plate 110 may be situated between thethird heating element 112 and the substrate 108 as illustrated inFIG. 1. The heat transfer plate 110 may enhance the spatial uniformityof the heat produced by the third heating element 112, resulting inlower spatial variation in the surface temperature of the substrate 108.The heat transfer plate 110 may be constructed of a thermally conductivematerial similar to the materials used to construct the first crucible102 including, but not limited to include graphite materials such aspurified pyrolytic grade graphite, metals, metals coated with ceramics,and any other known crucible construction material. In an aspect, theheat transfer plate 110 is constructed from a graphite material.

In an aspect, the stacked-source sublimation system 100 may additionallyinclude a third temperature sensor to monitor the third temperature ofthe substrate 108. The output of the third temperature sensor may beused as feedback to a control system used to control the third heatingelement 112 including, but not limited to: a proportional with integraland derivative (PID) controller or any other suitable controller.Non-limiting examples of suitable third temperature sensor devicesinclude: a thermocouple, an infrared temperature sensor, and apyrometer. In an aspect, the third temperature sensor is a pyrometer.

Any known heating device may be used as the third heating element 112without limitation. Non-limiting examples of devices suitable for use asa third heating element 112 include: resistive heating devices such asNiCr coils, inductive heating devices, and radiative heating devicessuch as quartz-halogen lamps. In one aspect, the third heating element112 may be one or more resistive NiCr coils. In this aspect, the NiCrwire may be embedded into the material of the heat transfer plate 110using an alumina-based ceramic material.

In an aspect, the third temperature may range from about 100° C. toabout 1000° C. during deposition. In another aspect, if the first sourcematerial 104 is CdTe and the second source material 118 is Mg, the thirdtemperature may range from about 300° C. to about 550° C. duringdeposition. In this aspect, the third temperature may be influenced bythe pressure at which the system 100 is operated as well as otherprocess parameters as described herein above.

d. Additional Crucibles, Heating Elements, and Manifolds

In various other aspects, the stacked-source sublimation system 100 mayincorporate additional elements to provide further enhancements to thecapabilities of the system 100 to provide the capability to produce awide variety of complex thin-film alloy layers. In one aspect, thesystem 100 may further include additional crucibles and associatedadditional manifolds. Each additional crucible defines an individualheated pocket within which an additional source material may be heatedto a temperature suitable for producing an additional flux bysublimation. The additional manifold associated with each additionalcrucible may transfer the additional flux from the additional heatedpocket to the first heated pocket 106. Within the first heated pocket106, the first flux, second flux, and any additional fluxes may becombined to produce a deposition mixture to be deposited on thesubstrate 108.

In this aspect, the additional crucible may be used to produce a varietyof ternary and quaternary thin film alloys. For example, if the firstsource material is CdTe, the second source material is Mg, and theadditional source material is Zn, the system 100 in this aspect may beused to produce a quaternary Cd_(1-x-y)Mg_(x)Zn_(y)Te alloy. Dependingon the configuration and utilization of flux control elements associatedwith each of the crucibles, the system 100 may be used to produce aternary Cd_(1-x)Mg_(x)Te ternary alloy transitioning to aCd_(1-y)Zn_(y)Te ternary alloy in an abrupt of graded fashion bysuccessively occluding/opening the manifold associated with the Znsource and concomitantly opening/occluding the manifold associated withthe Mg source. In another example, the manifold associated with the Znsource may be opened in one region and occluded in other regions of thesubstrate face, and the manifold associated with the Mg source may beopened in one region and occluded in other regions of the substrate faceto produce a Cd_(1-y)Zn_(y)Te composition in one region of the substrateface that gradually transitions to a Cd_(1-x)Mg_(x)Te composition inanother region of the substrate face.

The modular design of the additional crucibles allows the system 100 tobe configured to produce a wide variety of complex alloy layers with awide variety of spatial arrangements and variations in compositionthroughout the depth of the alloy layer and/or across the exposed faceof the substrate.

e. Integration into Manufacturing Systems

In various aspects, the stacked source sublimation system 100 may bescaled up or down as needed including, but not limited to scaling up tocommercial production scales. In an aspect, the system 100 may beintegrated into existing manufacturing equipment and processes toenhance the abilities of the equipment and processes to producematerials such as complex thin-film alloy layers.

For example, the system 100 may be integrated into a commercial devicethat incorporates additional manufacturing features including, but notlimited to: multiple pretreatment/deposition/annealing/post-treatmentstations, and automated positioning of the substrate at the multiplestations.

In one aspect, the stacked source sublimation system 100 may beintegrated into a manufacturing system 300 as illustrated schematicallyin FIG. 3. The stacked source sublimation system 100 may be situatedwithin the manufacturing system 300. In this aspect, the manufacturingsystem 300 may further include a substrate preheater 302, a load lock304 and a gate valve 306 for quick introduction of the substrate 108 andsource materials, and a magnetic transfer arm 308 to move the substrate108 from the substrate preheater 302 to the stacked source sublimationsystem 100 according to a predetermined schedule. The manufacturingsystem 300 may further include a diffusion pump 312 to maintain theinterior volume 310 at a predetermined base pressure that may be below10⁻⁵ Torr. The diffusion pump 312 may be outfitted with liquid nitrogenand water cold traps to prevent the inadvertent release of vaporproducts outside of the manufacturing system 300.

In other aspects, the manufacturing system 300 may further includeadditional process stations in any sequence and/or arrangement withoutlimitation to implement other manufacturing processes. Non-limitingexamples of additional process stations include: substrate cleaningstations, additional material deposition stations, halogen substancetreatment stations, annealing stations, back contact formation stations,substrate cooling stations, scribing stations, interconnection screenprinting stations, and any other suitable additional process stations.In an additional aspect, the stacked source sublimation system 100 maybe integrated into a manufacturing system similar to the systemdescribed in U.S. Pat. No. 6,423,565, which is incorporated by referenceherein in its entirety.

f. Compositions of Complex Thin-Film Alloys

In various aspects, the stacked-source sublimation system 100 may beused to deposit a variety of complex thin-film alloy layers on asubstrate 108. Non-limiting examples of complex thin-film alloycompositions that may be produced using the system include:Cd_(1-x)Mg_(x)Te, Cd_(1-x)Zn_(x)Te, Cd_(1-x)Mn_(x)Te, Cd_(1-x)Hg_(x)Te,Cd_(1-x)Bi_(x)Te, Cd_(1-x)Pb_(x)Te, Cd_(1-x)Mg_(x)S, Cd_(1-x)Zn_(x)S,Cd_(1-x)Mn_(x)S, and CdS_(x)Te_(1-x), and ZnMgTe.

In one aspect, the composition of the complex thin-film alloy layerproduced by the system 100 may be Cd_(1-x)Mg_(x)Te. In this aspect, theprocess parameters including but not limited to the heated pockettemperatures and the position of any flux control elements may bemanipulated to vary the proportions of Cd and Mg included in thethin-film alloy, as quantified by x. In this aspect, x may vary from 0(i.e. pure CdTe) to about 0.4-0.8. Cd_(1-x)Mg_(x)Te may become highlyhygroscopic and degrade in atmospheric conditions when x exceeds 0.4.

II. Methods of Using Stacked-Source Sublimation System

In various aspects, the stacked-source sublimation system 100 may beused to produce a thin-film alloy. A flow chart illustrating a method900 of depositing a thin-film alloy on a substrate in one aspect isprovided as FIG. 9.

a. Providing Stacked-Source Sublimation System

In this aspect, the method 900 includes providing the stacked-sourcesublimation system at step 902. The stacked-source sublimation system isdescribed herein previously and is illustrated in FIG. 1 in one aspect.In one non-limiting example, the stacked source sublimation system mayinclude: a first crucible defining a first heated pocket opening upwardat a pocket opening, a second crucible situated vertically below thefirst crucible and defining a second heated pocket; and a manifoldvertically situated between the first crucible and the second crucibleand connecting the second heated pocket to the first heated pocket.

b. Preparing Stacked-Source Sublimation System

In this aspect, the method 900 further includes preparing thestacked-source sublimation system at step 904. The first and secondsource materials may be loaded into the first and second crucibles ofthe system, respectively in an even spatial distribution as describedherein previously. The operational atmosphere may also be established atthis step as described herein previously. For example, an Argonatmosphere of about 40 mTorr may be established at step 904 in anaspect.

Also at step 904, the first and/or second source materials may be heatedto an elevated temperature to remove any residual oxidation layers fromthe surface of the source material pieces as described previously hereinabove. For example, if the source material is pure Mg, the Mg sourcematerial may be heated up to about 560° C. at step 904, and then reducedto a lower temperature during the deposition of the thin-film alloy inlater steps.

c. Situating and Preheating Substrate

The substrate may be situated within the system at step 906. Thesubstrate may be situated in a horizontal orientation with thedeposition surface facing downward above the pocket opening of the firstcrucible as described herein previously. In addition, the substrate maybe vertically aligned over the pocket opening at a vertical separationdistance as described previously herein. In an aspect, this verticalseparation distance may be at least about 1 μm.

At step 908, the substrate may be preheated and maintained at the thirdtemperature as described previously herein. In one aspect, thepreheating of the substrate may take place in a separate dedicatedstation, as described previously herein and illustrated in FIG. 3.Typically, the substrate is maintained at temperature that is lower thanthe first temperature at which the deposition mixture exiting the pocketopening of the first crucible is maintained.

d. Sublimating First Source Material

At step 910, the first source material may be sublimated in the firstcrucible at a first temperature to form a first flux as described hereinpreviously. The first temperature may be manipulated as described hereinpreviously to adjust aspects of the deposition process including, butnot limited to: the growth rate of the thin-film alloy layer and therelative inclusion of elements from the first flux within thecomposition of the thin film alloy layer.

e. Sublimating Second Source Material

At step 912, the second source material may be sublimated in the secondcrucible at a second temperature to form a second flux as describedherein previously. The second temperature may be manipulated asdescribed herein previously to adjust aspects of the deposition processincluding, but not limited to the relative inclusion of elements fromthe second source material within the composition of the thin film alloylayer.

f. Transferring Second Source Material

At step 914, the second flux formed at step 912 may be transferred fromthe second heated pocket to the first heated pocket as described hereinpreviously to form the deposition mixture. The deposition mixture isformed by the combination of the elements of the first flux and theelements of the second flux. In one aspect and as described hereinpreviously, the first heated pocket and second heated pocket may bedesigned to operate at a Knudsen number associated with an intermediateflow regime in order to facilitate the spatially uniform mixing of thefirst flux and second flux while inhibiting the formation of unwantednanoparticles.

In one aspect, the second flux may rise within the second heated pocketand passively move into the first heated pocket via the manifold, whichmay include a plurality of conduits, as described herein previously. Inanother aspect, adjustable flux control elements including but notlimited to adjustable flow valves, may be adjusted to regulate the rateof transfer of the second flux to the first heated pocket as describedherein previously. The adjustment of the flux control elements mayregulate aspects of the deposition process including, but not limited tothe relative inclusion of elements from the second source materialwithin the composition of the thin film alloy layer. In another aspect,the flux control elements may be used in addition to, or in lieu of, themanipulation of the second temperature within the second heated pocketas a means of controlling the composition of the thin-film alloy layer.

g. Depositing Thin-Film Alloy

In an aspect, the method 900 may further include depositing thethin-film alloy layer on the surface of the substrate at step 916. Inone aspect, the deposition mixture formed in the first heated pocketrises upward out of the pocket opening and contacts the surface of thesubstrate situated immediately above the pocket exit. Upon contact withthe cooler substrate surface, the deposition mixture condenses to formthe thin-film alloy layer.

The thickness of the thin-film alloy layer formed at step 916 may varydepending on the growth rate of the system and the duration of thedeposition at this step. In one non-limiting example, the system may becapable of growing a film with a depth of about 1 micrometer in abouttwo minutes. This growth rate may be influenced by any one or more of atleast several factors including, but not limited to the first and secondflux rates, the temperature of the substrate surface, as well as theduration of the deposition.

h. Reconfiguring System

The method 900 may further include reconfiguring the system at step 918.As described herein previously, the substrate may be removed to exposethe first heated pocket, resulting in the clearing of the contents ofthe first heated pocket. In one aspect, the reconfiguration of thesystem at step 918 may enhance the repeatability and uniformity of thethin-film alloy layers formed on successive substrates using this method900.

EXAMPLES

The following examples illustrate various aspects of the presentdisclosure.

Example 1 Optical Properties of Cd_(1-x)Mg_(x)Te Thin Film AlloyDeposited Using Prototype Stacked-Source Sublimation System

To demonstrate the feasibility of depositing a Cd_(1-x)Mg_(x)Te thinfilm alloy using a prototype stacked source sublimation system andmethod described herein above, the following experiments were performed.

A prototype stacked source sublimation system was situated within avacuum deposition system and used to deposit CdTe and Cd_(1-x)Mg_(x)Telayers on a glass substrate. The vacuum deposition system included amass flow controller that maintained a base pressure of 4×10⁻² Torr ofArgon through the deposition processes.

The prototype stacked source sublimation system 100 similar to thesystem described herein above and illustrated schematically in FIG. 1was used to perform the deposition experiments.

A CdTe layer was deposited on the substrate using the prototype system.The CdTe source material was sublimated in the upper crucible at atemperature of between about 500° C. and about 620° C. and rose upwardthrough the deposition pocket toward the substrate, which was maintainedat a temperature of between about 450° C. and about 520° C. The risingCdTe vapor formed into a polycrystalline layer upon contact with thecooler substrate.

In addition, a Cd_(1-x)Mg_(x)Te layer was deposited on the substrateusing similar conditions to those described above. In addition, the Mgpellets were sublimated in the lower crucible at a temperature ofbetween about 400° C. and about 560° C. after an initial heating up toabout 560° C. to remove any residual MgO that may have formed on the Mgpellets prior to deposition. The Mg vapor produced by sublimation wasintroduced into the deposition pocket via the manifold. The mixture ofMg vapor and CdTe vapor combined in the deposition pocket to form apolycrystalline Cd_(1-x)Mg_(x)Te layer upon contact with the glass/CdTesubstrate.

The optical properties of the CdTe layer and the Cd_(1-x)Mg_(x)Te layerwere analyzed using a commercially-obtained spectrophotometer. FIG. 2 isa graph summarizing the transmission spectra obtained for the twolayers, using the transmission spectrum of air as a baseline. Comparingthe CdTe spectrum to the CdMgTe spectrum shown in FIG. 2, a clear risein material band gap was observed in these transmission data, indicatingthe formation of the higher band gap Cd_(1-x)Mg_(x)Te film.

The results of these experiments demonstrated the feasibility ofdepositing a Cd_(1-x)Mg_(x)Te thin film alloy using the prototypestacked source sublimation system. A growth rate of up to 1.0 μm/min wasachieved using this prototype system.

Example 2 Optical Properties of Cd_(1-x)Mg_(x)Te Thin Film Alloys ofVarying Composition Deposited Using Prototype Stacked-Source SublimationSystem

To assess the optical properties of Cd_(1-x)Mg_(x)Te thin film alloys ofvarying compositions formed using a prototype stacked source sublimationsystem under a variety of process conditions, the following experimentswere performed.

The deposition of the Cd_(1-x)Mg_(x)Te thin film alloys was performedusing the prototype stacked source sublimation system describedpreviously in Example 1. The prototype stacked source sublimation systemwas situated within a manufacturing system similar to the manufacturingsystem described previously herein and shown schematically in FIG. 3. Abase pressure of below 10⁻⁵ Torr was accomplished using a diffusion pumpoutfitted with liquid nitrogen and water cold traps to prevent theinadvertent release of vapor products outside of the manufacturingsystem.

Glass/CdS substrates were formed prior to the current experiments by CSSdeposition of CdS onto TEC 10 solar-grade glass (Pilkington NorthAmerica, Inc.; Toledo, Ohio, USA). The Cd_(1-x)Mg_(x)Te layers weredeposited onto 3″×3″ glass/CdS substrates in an atmosphere of 20-60mTorr of Argon and at a substrate temperature of 450° C.-520° C. TheCdTe source and Mg source temperatures were specified independently tocontrol the growth rate and composition of the Cd_(1-x)Mg_(x)Te alloy.The CdTe source temperatures were varied between 540° C. and 600° C. tocontrol overall film growth rate. The Mg source temperature was variedbetween 380° C. and 520° C. to control the relative proportion of Mg inthe composition of the alloy. Typically, the Cd_(1-x)Mg_(x)Te filmsabout 1 μm were deposited in about 2 minutes, corresponding to adeposition rate of about 500 nm/min.

The optical properties of the Cd_(1-x)Mg_(x)Te layers were analyzedusing similar methods and equipment to those used in Example 1. FIG. 4is a graph summarizing the transmission spectra obtained for numerouslayers with various fractional Mg content. The spectra summarized inFIG. 4 indicated that Cd_(1-x)Mg_(x)Te layers with a range of band gapscould be produced by manipulation of the process conditions of theprototype stacked source sublimation system.

The transmission spectra were used to calculate the band gap of thefilms using a standard Tauc plot method. In addition, each layer wassubjected to planar Energy Dispersive X-ray Spectroscopy (EDS) using afield emission Scanning Electron Microscope (SEM) (model JSM-6500F; JEOLLtd., Tokyo, Japan) to quantify the atomic concentration of Cd, Te, andMg, as well as the ratio Mg/(Cd+Mg) for each layer.

The relationship between the Mg/(Cd+Mg) ratios and calculated band gapsof the layers is summarized in FIG. 5. A linear regression of theresulting data summarized in FIG. 5 had an R² value of greater than0.95, indicating the layers films are of consistent quality. Further,the data and regression indicated a band gap of about 1.5 eV when Mg wasabsent from the layer (x=0). This value of 1.5 eV is consistent with theknown band gap of similar untreated CdTe films.

The results of this experiment indicated that Cd_(1-x)Mg_(x)Te thin filmalloys with a range of band gaps could be reliably produced using theprototype stacked source sublimation system with controlled variation ofprocess conditions.

Example 3 Crystallinity of Cd_(1-x)Mg_(x)Te Thin Film Alloys of VaryingComposition Deposited Using Prototype Stacked-Source Sublimation System

To assess the layer crystalline structure of Cd_(1-x)Mg_(x)Te thin filmalloys of varying compositions formed using a prototype stacked sourcesublimation system under a variety of process conditions, the followingexperiments were performed.

The Cd_(1-x)Mg_(x)Te layers produced using the methods of Example 2 werefurther subjected to SEM imaging using the SEM microscope described inExample 3. FIGS. 6A-C are SEM images of selected Cd_(1-x)Mg_(x)Telayers: a layer with x=0 and a band gap (E_(g)) of 1.494 (FIG. 6A); alayer with x=0.156 and E_(g)=1.789 (FIG. 6B); and a layer with x=0.0.346and E_(g)=2.125 (FIG. 6C). The SEM images indicated a consistent patternof change in layer crystallinity with increased Mg content. The crystalgrain size decreased with increased Mg content in the layers.Additionally, the grains of the higher band gap films (higher Mgcontent) appeared to have more dislocations and structural defectscompared to the layers with lower band gaps.

The Cd_(1-x)Mg_(x)Te layers were also subjected to X-Ray Diffraction(XRD) measurements utilizing Cu Kα radiation (λ=1.5418 Å) (model D-8Discover; BRUKER AXS, Inc., Madison, Wis., USA) to quantify grainorientation. The XRD spectra measured from the layers shown in FIGS.6A-C are summarized in FIG. 7. For all layers the (111) peak had a broadshoulder at lower 2θ values, which is typically associated withintermixing between CdS and Cd_(1-x)Mg_(x)Te. This evidence ofintermixing is likely due to the relatively high substrate temperatureduring deposition and low film thickness of less than 1 μm.

The results of these experiments indicated that as the concentration ofMg increased in the Cd_(1-x)Mg_(x)Te layers produced using the prototypestacked source sublimation system, the crystal grain size decreased andthe occurrence of dislocations and structural defects increased.

The foregoing merely illustrates the principles of the technologydisclosed herein. Various modifications and alterations to the describedembodiments will be apparent to those skilled in the art in view of theteachings herein. It will thus be appreciated that those skilled in theart will be able to devise numerous systems, arrangements and methodswhich, although not explicitly shown or described herein, embody theprinciples of the disclosed technology and are thus within the spiritand scope of the disclosed technology. From the above description anddrawings, it will be understood by those of ordinary skill in the artthat the particular embodiments shown and described are for purposes ofillustrations only and are not intended to limit the scope of thedisclosed technology. References to details of particular embodimentsare not intended to limit the scope of the disclosed technology.

What is claimed is:
 1. A method for the deposition of a complexthin-film alloy on a substrate, the method comprising: providing astacked-source sublimation system comprising a first crucible defining afirst heated pocket opening upward at a pocket opening; and a secondcrucible operatively connected to a second heating element and defininga second heated pocket opening upward into a manifold comprising aplurality of conduits, wherein the plurality of conduits connects thesecond heated pocket to the first heated pocket and the plurality ofconduits is distributed uniformly over a bottom wall of the first heatedpocket and each of the plurality of conduits extends vertically upwardto connect the second heated pocket to the first heated pocket throughthe bottom wall; situating the substrate vertically above the pocketopening at a vertical separation distance of at least about 1 μm;sublimating a first source material at a first temperature within thefirst heated pocket to form a first flux; sublimating a second sourcematerial at a second temperature within the second heated pocket to forma second flux; transferring the second flux from the second heatedpocket to the first heated pocket via the manifold to form a depositionmixture; and contacting the deposition mixture with the substrate todeposit the complex thin-film alloy on the substrate, wherein thesubstrate is maintained at a third temperature.
 2. The method of claim1, wherein: the first source material is a compound chosen fromtelluride compounds, sulfide compounds, chloride compounds, and selenidecompounds; and the second source material is a metal or a compoundchosen from telluride compounds, sulfide compounds, chloride compounds,and selenide compounds.
 3. The method of claim 2, wherein: the metal ischosen from Mg, Zn, Mn, Cu, Hg, Bi, Pb, and Cd; the telluride compoundsare chosen from CdTe; the sulfide compounds are chosen from CdS; and thechloride compounds are chosen from CdCl₂, CuCl₂, and MgCl₂.
 4. Themethod of claim 3, wherein: the first source material consists of CdTeand the second source material consists of Mg; the first temperatureranges from about 540° C. to about 620° C.; the second temperatureranges from about 350° C. to about 520° C.; and the third temperatureranges from about 300° C. to about 550° C.
 5. The method of claim 1,wherein the first flux is controlled by manipulating the firsttemperature of the first heated pocket and the second flux is controlledby manipulating the second temperature of the second heated pocket. 6.The method of claim 5, wherein: the second flux is further controlled bya flux control element operatively connected to the manifold; and theflux control element is chosen from: an adjustable shutter platesituated across each conduit cross section of the plurality of conduits,wherein the shutter plate occludes the plurality of conduits whenadjusted to a closed shutter position and the shutter plate opens theplurality of conduits when adjusted to an open shutter position; and aplurality of adjustable valves operatively connected to the plurality ofconduits of the manifold, wherein the plurality of valves occludes theplurality of conduits when adjusted to a closed valve position and theplurality of valves opens the plurality of conduits when adjusted to anopen valve position.
 7. The method of claim 5, wherein: the rate ofdeposition of the complex thin-film alloy on the substrate is controlledby manipulating the first flux; and the composition of the complexthin-film alloy deposited on the substrate is controlled by manipulatingthe second flux.
 8. The method of claim 1, wherein the method isconducted within an operational atmosphere at a pressure ranging fromabout 0.1 mTorr to about 100 mTorr.